The goal of this proposal is to understand the mechanism and developmental process in plants. The focus is on the Activator (Ac)/Dissociation (Ds) family of transposons of Zea mays. These elements are members of a large superfamily of elements that transpose via DNA intermediates. We propose a genetic and biochemical approach to dissect the transposition process to identify requisite cis-acting elements and trans-acting factors. These studies exploit both maize developmental genetics and the ability of Ac/Ds clement to function in the heterologous host, Arabidopsis. Our specific aims are to: i) identify cellular factors required for transposition using saturation mutagenesis including enhancers and suppressor mutations that affect transposition processes; ii) initiate biochemical studies to identify interacting protein factors; iii) investigate a transposon-inactivation process associated with methylation and repeat-induced gene silencing; iv) study how mutations affect the frequency and timing of transposition during development; and v) (attempt to develop in vitro transposition assays to study target site specificity and the biochemical requirements for transposition. These studies should lead us to a better understanding of the mechanism and developmental control of transposition and provide a greater understanding of gene silencing and chromosomal position effects. Studies on transposition will also shed light on the health-related processes of spontaneous mutations, genetic diseases, somatic mutations and genome structure.